This invention relates to calcium aluminate compositions, articles made from these compositions and methods of making such articles. More particularly, the invention relates to low thermal expansion calcium aluminate compositions and articles and methods of making low expansion articles.
Low thermal expansion ceramic bodies are desirable in a wide variety of applications. For example, low expansion bodies have been used as filters for fluids, in particular, as diesel particulate filters and as substrates for catalytic converters, an example of which is known in the art as a honeycomb substrate. Additionally, low thermal expansion bodies are desirable in applications where the thermal shock resistance and the ultimate use temperature are high. Substrates used under conditions of high thermal gradients are examples of this application. For example, structures such as honeycomb and cellular substrates are subjected to harsh environments, which require high thermal shock resistance, low thermal expansion, and high mechanical shock properties. Maintaining these properties for extended periods of time in their intended environments eliminates many potentially useful refractory materials.
Cordierite substrates, typically in the form of a honeycomb body, have long been preferred for use as substrates to support catalytically active components for catalytic converters on automobiles, in part due to high thermal shock resistance of cordierite ceramics. The thermal shock resistance is inversely proportional to the coefficient of thermal expansion. That is, honeycombs with a low thermal expansion have a good thermal shock resistance and can survive the wide temperature fluctuations that are encountered in the application. Manufacturers work continuously to optimize the characteristics of cordierite substrates to enhance their utility as catalyst carriers. Specifically, manufacturers continually strive to optimize the thermal shock resistance and other properties of the cordierite substrates.
Another property of certain types of catalysts is the ability to purify exhaust gases and the ability to convert carbon monoxide, hydrocarbons and nitrogen oxides (NOx) produced during engine operation into less environmentally harmful gases. Some catalyst systems utilize alkali metals contained on the catalyst support store nitrogen oxides, and such catalysts are referred to in the art as NOx adsorbers. A disadvantage of presently available catalyst and purification systems is that most of the alkali metals contained on the catalyst support for storing NOx readily react with cordierite within the temperature range of interest for NOx adsorber use. For example, potassium, a widely used alkali adsorber material, appears to readily react with cordierite, which draws the potassium out of the high surface area washcoat and prevents it from performing its adsorber function. Furthermore, the potassium reacts with cordierite to form relatively high coefficient of thermal expansion (CTE) phases that make the substrate and the catalyst system much less thermal shock resistant.
There is a need to provide alternative low CTE materials that are useful in high temperature applications. It would be desirable to provide materials that have a low CTE and excellent thermal shock resistance.
One embodiment of the invention relates to a ceramic article comprised of calcium aluminate including a main phase of CaAl4O7 and minor phase of CaAl2O4, and which exhibits a thermal expansion of less than about 25xc3x9710xe2x88x927/xc2x0 C. over the temperature range of about 25xc2x0 C. to 800xc2x0 C. Other embodiments relate to a calcium aluminate article having a thermal expansion less than about 20xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. Still other embodiments relate to a calcium aluminate article having a thermal expansion less than 15xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. Other embodiments relate to calcium aluminate articles having a thermal expansion less than about 10xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C., and alternative embodiments relate to calcium aluminate articles having thermal expansions less than about 5xc3x9710xe2x88x927/xc2x0 C. over the temperature range 25xc2x0 C. to 800xc2x0 C. In certain embodiments, the articles contain a network of microcracks and include grains having a median grain size of between about 10 microns and 100 microns. The articles of the present invention can be used in a wide variety of applications, including but not limited to manufacturing honeycomb substrates for high temperature applications and honeycomb substrates for NOx adsorption applications.
Other embodiments of the invention relate to methods of manufacturing a calcium aluminate article which exhibits low thermal expansion. In one embodiment, the method includes mixing CaO and Al2O3 source powders, forming a shaped article from the powders and heating the article to a temperature exceeding about 1500xc2x0 C. to form an article containing between 21.6% and 30% by weight CaO. In some embodiments, the article is fired to a temperature such that a network of microcracks and grains sized between 10 microns and 100 microns are present in the article after firing.
It is to be understood that both the foregoing general description and the following detailed description are exemplary and are intended to provide further explanation of the invention as claimed.